If your WordPress blog is hosted at wordpress.com, you cannot insert adsense code into your posts.
If your blog is hosted at your Web site, you add the adsense HTML in one of the theme pages, such as index.php, sidebar.php, or header.php. From the Admin Panel, go to Presentation-Theme Editor. Insert your Adsense code outside of the WordPress tags. You only need to do this once and it works for all posts. This is far better than having to add the Adsense code for every post, which won’t work anyway because WordPress reformats your entry to make it conform to HTML standards.
There’s a nice article at problogger.net that illustrates how to place adsense code after, say, 3 posts.
Another option is to use a WordPress plugin. A number of these exist – I use Adsense Deluxe. There are quite a few others. With a plugin, you can control where and when you place adsense code. In addition, (most of the time) the plugin does not change the code for the theme, which is a Good Thing. Check out the WordPress plugin database for others.
